I. Field of the Invention
This invention relates generally to a diagnostic instrument for assessing lung function, and more particularly to a small, lightweight, on-airway instrument for assessing pulmonary performance of a patient.
II. Description of the Prior Art
Pulmonary function testing is a valuable tool for evaluating a patient's respiratory system. Pulmonary function tests (PFTs) is a generic term used to indicate a battery of studies of maneuvers that may be performed using standardized equipment to measure lung function. PFTs may include simple screening spirometry, formal lung volume measurement, diffusing capacity for carbon monoxide and arterial blood gases.
PFTs are noninvasive diagnostic tests that provide measurable feedback about the function of the lungs. By assessing lung volumes, capacities, rates of flow and gas exchange, PFTs provide information that, when evaluated by a medical professional, can help diagnose certain lung disorders.
A normally-functioning pulmonary system operates on many different levels to insure adequate balance. One of the primary functions of the pulmonary system is ventilation, i.e., the movement of air into and out of the lungs.
Various medical conditions can interfere with ventilation. Such conditions are typically classified as being “restrictive” or “obstructive”. An obstructive condition occurs when air has difficulty flowing out of the lungs due to resistance, causing a decrease flow of air. A restrictive condition occurs when the chest muscles are unable to expand adequately, creating a disruption in airflow. PFTs involve several different procedures for obtaining values that can be compared to standards for a large population for comparison purposes. Some of the more common values typically measured during PFT include:
PARAMETERDESCRIPTIONUNITFVCForced Vital CapacityLFEV1Forced Expiratory Volume in 1 sLFEV1/FVCFEV1 in % of FVC%PEFPeak Expiratory FlowL/sFIV 1Forced Inspiratory Volume in 1 sLFRCFunctional Residual CapacityLDLCODiffusing CapacityMIPMaximum Inspiratory PressureMMHgMEPMaximum Expiratory PressureMMHg
The spirometry measurements typically require a voluntary maneuver in which a patient inhales maximally from vital respiration to total lung capacity and then rapidly exhales to the fullest extent until no further volume is exhaled at residual volume. This maneuver may be performed in a forced manner to generate forced vital capacity (FVC) or in a more relaxed manner to generate a slow vital capacity (SVC). Various types of spirometers are known in the art for directly measuring the volume of air displaced or that measures airflow by a flow-sensing device, such as a pneumotachometer. Presently, most PFT laboratories use a microprocessor-driven pneumotachometer to measure airflow directly and from which volume can be mathematically derived. Because spirometry is an expiratory maneuver, it measures exhaled volume or vital capacity, but does not measure residual volume, functional residual capacity (resting lung volume), or total lung capacity. Vital capacity is a simple measure of lung volume that is usually reduced in patients suffering from restrictive disorders.
Other pulmonary function tests are needed to measure total lung capacity, which is derived from the addition of functional residual capacity to inspiratory capacity obtained from spirometry. FRC is usually measured by a gas dilution technique or body plethysmography. Gas dilution techniques are based on a simple principle, are widely used and provide a good measurement of all air in the lungs that communicates with the airways. Lung volume measurements obtained using gas dilution techniques either used closed-circuit helium dilution or open-circuit nitrogen washout. In the nitrogen-washout technique, a patient is made to breathe 100% oxygen until all of the nitrogen theretofore in the lungs is washed out. The exhaled volume and the nitrogen concentration in that volume are measured. The difference in nitrogen volume at the initial concentration and the final exhaled concentration allows a calculation of the intrathoracic volume, FRC.
Another important factor in carrying out a PFT is to assess the diffusing capacity of the lungs. It is a measure of the ability of the lungs to transfer oxygen to the blood and to remove CO2 therefrom.
Generally speaking, all methods for measuring diffusing capacity in clinical practice today rely on measuring the rate of carbon monoxide (CO) uptake and estimating carbon monoxide driving pressure. The most widely used and standardized technique is the single-breathe breath-holding technique. In this procedure, a patient inhales a known volume of test gas that usually contains 0.3% methane, 0.3% carbon monoxide, 21% oxygen and the remainder nitrogen. This test gas is inhaled and the patient holds his/her breath for 10 seconds. The patient then exhales to first wash out the mechanical and anatomic dead space and then an alveolar sample is collected. DLCO is then calculated from the total volume of the lung, the breath hold time and the initial and final alveolar concentrations of CO. The exhaled methane concentration is used to calculate a single breath estimate of total lung capacity and initial alveolar concentration of CO.
Almost all current systems for performing PFTs and especially lung volume and lung diffusion capability use a “side streaming” sensor apparatus in which respiratory gas samples are drawn from an airway mouthpiece and conveyed through a sampling tube to a sensor located distant from the sampling site, usually with the aid of a vacuum pump. When it is recognized that lung diffusion measurements (DLCO) involve only small concentrations, it is necessary to have reasonably long path lengths in a non-dispersive infrared (NDIR) system to yield good resolution down to, say, 30 ppm. In that both CO and CH4 are weak absorbers of infrared energy, it is therefore impossible to obtain any reasonable absorption differences in the absence of a long path length. Also in conducting a DLCO measurement, the speed of response is a major factor to obtain good integrals for the gaseous volumes. A very high flow is needed to purge down the side streaming system if a reasonable response is to be achieved. A flow of several hundred cubic centimeters per minute is typically required and this removes a high volume of gas from the patient's circuit, thus creating an artificial breathing environment for the patient. In addition, with this high flow, it is necessary to create quite high vacuums downstream of the analyzer to pull this amount of flow through small bore lines which are necessary to preserve waveform. This results in a loop pressure much below atmospheric. This low pressure directly affects the partial pressure of the gases being measured, CO and CH4, and further reduces resolution.
The “side streaming” type of measurement apparatus also involves issue relating to water removal, temperature and humidity differences at the sampling site and measurement/sensing site, unwanted mixing of a current sample with previous samples as the current sample is being drawn through the sample tube, variability in the pressure drop across the tubing and the introduction of a phase delay between the sample time and the measurement/sensing of that sample. The phase delay is attributable to the fact that flow is measured instantaneously at the patient's mouth with a pneumotach and the measured pressure differential from the pneumotach is transmitted at the speed of sound. The analysis for a component level at any given flow necessarily lags behind the flow and has to be allowed for in the calibration and software. To reduce the lag, the flow can be increased but this results in higher vacuum in the cell and more patient interference.
More recently, efforts have been made to develop “mainstream” respiratory gas sensors where the measurement/sensing takes place immediately adjacent the subject's mouthpiece, i.e., directly on the airway as a way of obviating the aforementioned problems with sidestream systems. It is accordingly a principal purpose of the present invention to provide an improved mainstream analyzer involving an on-airway construction that allows for a relatively long sample chamber path length and much greater resolution than can be achieved with side streaming systems and which does so without sacrificing pressure drop and distortion of the patient circuit.
It is a further object of the invention to provide an improved mainstream respiratory gas analyzer system offering an improved speed of response over what can be achieved using side streaming.
Yet another object of the invention is to provide a mainstream respiratory gas analyzer in which water vapor is managed rather than removed.